The non-invasive measuring of certain physiological properties has long been practiced by physicians in the prior art. Such techniques include the transmitting of one or more wavelengths of radiation into perfused tissue and detecting the amount of light passing through or being reflected from the tissue, and using the signal obtained to determine various physiological measurements such as arterial oxygen saturation and heart rate. Examples of prior art devices are illustrated in U.S. Pat. Nos. 4,485,464; 4,086,915; and 3,847,483. The sensors used with such devices are either of the transmissive type, i.e., light being transmitted to a sensor on the opposite side of the tissue, or of the reflective type, wherein the radiation sensed is reflected off the tissue.
A common problem for transmissive and reflectance type sensors is the amount of pressure applied by the sensor against the tissue. It is important the sensor be pressed firmly against the tissue to efficiently use the radiation being transmitted and minimize radiation leakage problems with the sensor. However, too much pressure will cause blood to leave the tissue making it more difficult to obtain accurate measurements. Thus, a delicate balance is needed in order to optimize performance of the sensor. Additionally, patient comfort during prolonged use of the sensor becomes important.
A further problem with transmissive type sensors is that they require the light to pass through the tissue and are thus limited to contain parts of the body such as a digit or an ear. Further, since transmissive type sensors require the radiation detector to be located opposite the radiation source, proper sensor alignment is very critical for optimal operation. Additionally, radiation leakage at the radiation source and sensor can lead to significant measurement errors.
With regard to reflective type sensors, it is also very important that the radiation emitting source be in relatively close proximity to the radiation detector. However, this close proximity presents the risk of radiation leakage from the radiation source to the radiation detector without passing through pulsatile tissue, and therefore provide inaccurate readings. Conformance of the radiation source and detector with the tissue is very important as improper sensor alignment may cause errors in measurement. Further, if insufficient conformance of the detector radiation source exists, the sensor may not operate at all.
Applicants have invented a reflective type sensor which minimizes or avoids the many problems of the prior art.